Studies in mice that are genetically defective in various mismatch repair proteins strongly suggest that mismatch repair (MMR) plays a major role in the generation of antibody diversity through its affect on somatic hypermutation (SHM) of antibody variable region genes and on class switch recombination (CSR). In addition, deficiencies in MMR predispose to B and T cell malignancies. Studies with of mice MSH2 and MSH6 deficiencies have lead to the hypothesis that SHM and perhaps CSR occur in two phases: one that is AID dependent and results in mutations in G and C in hotspots and a second that depends upon MMR and results in mutations in all bases and is not restricted to hot spot motifs. I propose to test this hypothesis and learn more about the role of MMR by examining mice that are defective in exonuclease 1 or are expressing MMR proteins that have mutations that inactivate their ability to bind ATP. It is possible that the phenotypes of mice lacking MMR activity is partly the reflection of positive and negative selection for B cells making higher affinity or self reactive antibodies. I will therefore also study the role of mismatch repair and dissect out the impact of individual mismatch repair protein by inactivating each of the known MMR proteins and expressing mutant proteins in antibody-forming cells in culture where the positive and negative selection of mutated antibodies will not occur. I will use the MMR repair deficient cells to search for additional proteins that are involve in SHM. These studies should provide new insights into the biochemical mechanisms of SHM and CSR and the role of MMR in predisposing to B cell malignancies and other cancers. [unreadable] [unreadable]